1. Technical Field of the Invention
The present invention relates generally to an improved structure of a spark plug suitable for downsizing, and more particularly to such a spark plug designed to ensure the stability in location of production of sparks between a center and a ground electrode.
2. Background Art
Typical spark plugs for automotive engines or gas engines are equipped with a center electrode and a ground electrode. The center electrode is disposed within a metal shell and has a tip exposed outside the metal shell. The ground electrode is joined at one end thereof to the metal shell and bent to have the other end thereof face the center electrode through a spark gap.
U.S. Pat. No. 6,653,767 B2 to Morita et al., assigned to the same assignee as that of this application, discloses a small-sized spark plug equipped with cylindrical chips joined to center and ground electrodes. The cylindrical chips have end surfaces which are opposed to each other through a spark gap. Each of the cylindrical chips has a diameter of is 1.1 mm or less, namely, a cross sectional area of 0.95 mm2. The spark plug has the decreased spark gap, however, is designed to ensure the ignitability of fuel without sacrificing the growth of flame kernel within the engine.
The inventors of this application have analyzed the structure of spark plugs suitable for downsizing and found problems, as discussed below.
FIG. 19 is a partial side view which shows an example of a spark plug of the type as described above.
The center electrode 30 is disposed within the metal shell 10 with the tip 31 exposed outside the metal shell 10. The ground electrode 40 is welded at an end 42 to the metal shell 10 and opposed at an end 41 to the tip 31 of the center electrode 30.
The cylindrical chips 35 and 45 are joined to spark gap-facing portions 31 and 43 of the center and ground electrodes 30 and 40. The cylindrical chips 35 and 45 have end surfaces 35a and 45a opposed to each other through the spark gap 50.
Each of the cylindrical chips 35 and 45 is made of, for example, Ir (Iridium) or Pt (Platinum). The cylindrical chips 35 and 45 are located to have longitudinal center lines 35b and 45b aligned with each other.
Application of voltage across the center and ground electrodes 30 and 40 will result in an increased strength of electric field at corners or edges of the cylindrical chips 35 and 45. This causes sparks to appear, as indicated by arrows in FIGS. 19 and 10, between edges of the end surfaces 35a and 45a of the cylindrical chips 35 and 45, not flat portions thereof.
The inventors have found that such sparks are produced at random on different locations of the edges of the end surfaces 35a and 45a, thus resulting in a variation in velocity of combustion between combustion cycles.
A portion P6 of the edge of the end surface 45a of the cylindrical chip 45 of the ground electrode 40 close to the bend 44 is an area from which much thermal energy is withdrawn to the metal shell 10, so that the temperature thereof is kept lower, while a portion P10 of the edge of the end surface 45a of the cylindrical chip 45 close to the tip 41 is an area from which less thermal energy is withdrawn to the metal shell 10, so that the temperature thereof is kept higher.
Therefore, when sparks are produced on the portion P10 of the end surface 45a of the cylindrical chip 45 close to the tip 41, it will results in quick growth of flame kernel in the engine and increased velocity of combustion. Conversely, when sparks are produced on the portion P6 of the end surface 45a close to the bend 44, it will result in slow growth of flame kernel and decreased velocity of combustion.
Specifically, conventional spark plugs of the type, as described above, are subjected to a variation in velocity of combustion between combustion cycles of, for example, automotive spark ignition engines. This results in a variation in amount of thermal energy produced between the combustion cycles, which leads to mechanical vibrations of the engine.